The invention relates to the production of bimetallic ethylene polymerization catalysts systems containing two sources of transition metals for the production of broad and/or bimodal molecular weight distribution (MWD) polyethylene resins in a single reactor. In a preferred embodiment, the invention relates to the production of titanium/zirconium-based bimetallic catalyst systems that produce broad and/or bimodal MWD polyethylene resins in a single reactor. The low molecular weight (LMW) polymer component in the resin is produced by the Zr active centers, while the high molecular weight (HMW) polymer component is produced by the Ti active centers. The relative productivity of the two active centers determines the ratio of the HMW and the LMW polymer components in the final resin.
This invention particularly relates to a new procedure for preparing bimetallic catalysts. This procedure results in bimetallic catalysts with a more uniform inter-particle distribution of the metals which produce the HMW and LMW polymer components.
The uniform inter-particle distribution of the metals in the catalyst results in the reduction of gel particles in polyethylene film. Gel particles are attributable to high molecular weight polymer components which are substantially greater in molecular weight than the surrounding matrix resin. The presence of gels in polyethylene film interfere with the film-forming process, reduce film toughness properties and lower the film quality rating (FQR) and, hence, must be avoided.
This invention relates to supported bimetallic ethylene polymerization catalysts with improved inter-particle distribution of the Zr active centers and facilitates the scale-up production of the catalyst. This invention also includes a new procedure for preparing bimetallic catalysts.
The invention relates to the production of bimetallic catalyst systems for ethylene polymerization containing two transition metals. These catalysts produce broad/bimodal MWD polyethylene resins in a single reactor. In a preferred embodiment, the invention relates to the production of titanium/zirconium-based bimetallic catalyst systems that produce broad/bimodal MWD polyethylene resins in a single reactor. The LMW polymer component in the resin is produced by the Zr active centers, while the HMW polymer component is produced by the Ti active centers. The relative productivity of the two active centers determines the ratio of the HMW and the LMW polymer components in the final resin.
This invention includes a new procedure for preparing bimetallic catalysts. Herein, bimetallic (containing two transition metals) catalyst precursors are produced without isolating the titanium component prior to introduction of the zirconocene component. Activated bimetallic catalyst precursors exhibit good productivity and produce resins with a bimodal MWD. The low molecular weight polymer component in the resin is produced by the zirconocene active centers while the high molecular weight polymer component is produced by the Ti-based centers. Typically, the bimetallic catalyst precursor is prepared in two steps. First, the titanium component is prepared and isolated as a free-flowing powder. Then the zirconium component is added to the titanium component to form the final bimetallic catalyst precursor. The one-stage incorporation of each of the two metals onto a support to produce the bimetallic catalyst precursors of this invention greatly reduces the batch time of the catalyst preparation. Moreover, in preferred bimetallic catalysts prepared according to this invention, the zirconium catalyst component is contacted with a trialkylaluminum compound such as trimethylaluminum (TMA) or triethylaluminum (TEAL) prior to addition to the titanium catalyst component. This step is carried out in the absence of methylalumoxane (MAO). Bimetallic catalysts prepared with this procedure have significantly higher activity than catalysts prepared without this particular step.